330 



Fishery Bulletin 101(2) 



data become available, it could be worthwhile to include 

 them in the prior for a, although there is no practical value 

 in including the data currently available. 



Status of winter-run Chinook salmon 



Although not the primary purpose of this study, our 

 model does provide an assessment of the present status 

 of winter-run chinook salmon: the quasi-extinction prob- 

 ability of 28% within 50 years indicates that winter-run 

 chinook salmon face a substantial extinction risk, in spite 

 of the probable improved survival since the ESA listing. 

 The ESA does not specify quantitative risk levels corre- 

 sponding to threatened or endangered status, but under 

 the World Conservation Union's Red List extinction risk 

 criteria (lUCN, 1994), winter-run chinook salmon would 

 be classified as "vulnerable" (>10'7f extinction probability 

 in 100 years). Winter-run chinook salmon extinction risk 

 is higher than the 10% probability of extinction in 50 years 

 specified as "safe" by Botsford and Brittnacher ( 1998). Fur- 

 thermore, the true quasi-extinction risk is probably higher 

 than indicated by our analysis because we have neglected 

 some sources of risk that could be significant at population 

 levels in excess of the quasi-extinction threshold, such as 

 catastrophic events. 



Botsford and Brittnacher ( 1998) developed a somewhat 

 similar model of winter-run chinook salmon spawning 

 escapement that predicts almost certain extinction for 

 winter-run chinook salmon in the absence of increased sur- 

 vival. The differences between the results presented here 

 and those of Botsford and Brittnacher ( 1998) illustrate the 

 importance of including parameter uncertainty and allow- 

 ing for time-varying population growth rate. Their model 

 assumed constant mean population growth rate, whereas 

 ours allowed for a change (J) in the population growth rate 

 following the conservation measures initiated in 1989. The 

 more optimistic prediction in this paper derives mostly 

 from the substantial probability that population growth 

 rate increased following implementation of conservation 

 measures. This can be illustrated by setting A to zero and 

 refitting our model. The quasi-extinction probability with 

 zl = is 69%. Much of the remaining discrepancy between 

 our results and those of Botsford and Brittnacher (1998) 

 arises from including parameter uncertainty, which allows 

 for the possibility that population growth might be higher 

 than its maximum likelihood estimate. The predicted de- 

 cline of the adult striped bass population from 700,000 to 

 512,000 contributes a smaller effect to increased survival 

 probability than does the effect of conservation measures. 

 Both analyses are similar, however, in that they indicate 

 winter-run chinook salmon face significant extinction risk 

 and require further con.servation action. 



Acknowledgments 



This work benefited from participation by STL in the 

 Predicting Extinction Working Group supported by the 

 National Center for Ecological Analysis and Synthesis, 

 a Center funded by NSF (grant no. DEB-94-21535), 



the University of California at Santa Barbara, and the 

 State of California. The authors thank L. Goldwasser 

 and E. Bjorkstedt for comments on an earlier version of 

 this manuscript, and J. Emlen and M. Prager for critical 

 reviews of the final version. Any remaining errors are the 

 responsibility of the authors. 



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